1. Field
This application relates generally to wireless communication and more specifically, but not exclusively, to time tracking and/or frequency tracking.
2. Introduction
A wireless communication network may be deployed over a geographical area to provide various types of services (e.g., voice, data, multimedia services, etc.) to users within that geographical area. In a typical implementation, macro access points (e.g., each of which provides service via one or more macro cells) are distributed throughout a macro network to provide wireless connectivity for access terminals (e.g., cell phones) that are operating within the geographical area served by the macro network.
As the demand for high-rate and multimedia data services rapidly grows, there lies a challenge to implement efficient and robust communication systems with enhanced performance. To supplement conventional network access points (e.g., to provide extended network coverage), small-coverage access points (e.g., low power access points) may be deployed to provide more robust indoor wireless coverage or other coverage to access terminals inside homes, enterprise locations (e.g., offices), or other locations. Such small-coverage access points may be referred to as, for example, femto cells, femto access points, home NodeBs, home eNodeBs, or access point base stations. Typically, such small-coverage access points are connected to the Internet and the mobile operator's network via a DSL router or a cable modem. For convenience, small-coverage access points may be referred to as femto cells or femto access points in the discussion that follows.
Typically, femto cells transmit their signals with a certain time and frequency accuracy as mandated by the relevant air interface specification. For example, in cdma2000 systems, all access points (base stations) are required to be synchronized to the “system time.” This “system time” is synchronous to coordinated universal time (UTC) (except for leap seconds) and uses the same time of origin as global positioning system (GPS) time, within some small error.
Time and frequency synchronization amongst different access points of the network is required for several purposes including, for example, controlling inter-access point interference (which would otherwise arise if different access points transmitted with widely different frequencies), and ensuring successful hand-off of an access terminal (mobile station) from one access point to another. If a femto cell is able to track the timing of nearby macro cells, the femto cell may coordinate its beacon transmissions with the wake-up time of the access terminals that are camped on the macro cell. This allows efficient femto cell idle-mode discovery and reduces the interference that would otherwise be induced at macro access terminals as a result of the femto cell beacon transmissions.
Current techniques used by femto cells for time and frequency tracking include deriving timing from a GPS receiver, deriving timing from a central accurate clock using Internet Protocol (IP) techniques such as IEEE1588, deriving timing from terrestrial TV broadcasts, and sniffing signals from neighboring macro access points. However, these techniques have several drawbacks. For example, a GPS receiver is not ideal for a low-cost consumer device such as a femto cell. Furthermore, a GPS signal may not be available in typical femto cell deployment scenarios such as inside buildings, basements, warehouses, etc. To avoid some of the drawbacks inherent to the use of GPS-based timing, a femto cell may instead rely on neighboring macro access points for time and frequency synchronization. In this case, a femto cell sniffs a neighboring macro access point's forward link (FL) transmissions (e.g., using a special module known as Network Listen Module) and uses the FL waveform structure as well as messages sent by the macro access point to derive timing and frequency information. However, to sniff neighboring macro access points that are transmitting in the same frequency band/channel as the femto cell, the femto cell transmitter is shut down so that femto cell forward link (FL) transmission do not interfere with the ability of the femto cell to receive macro access point FL transmissions. Therefore, such a Network Listen Module-based time and frequency tracking technique is used only when there is no user currently being serviced by the femto cell (e.g., there is no active voice/data session on-going on the femto cell) on the carrier frequency (and potentially adjacent carrier frequencies) to be sniffed by the Network Listen Module. In view of the above, there is a need for more efficient and reliable mechanisms for providing time and frequency synchronization for access points.